Chapter 48: The Nuclear Fuel Cycle

This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on April 22, 2013.

Nuclear fuel is defined as the fissionable nuclear material in the form of fabricated elements for loading into the reactor core of a nuclear power plant and the nuclear fuel cycle is referred to the various activities associated with the production of electricity from nuclear reactors.  The cycle also includes the processes for dealing with spent nuclear fuel which is classified as radioactive waste.

1.      RADIOACTIVE WASTE WITHIN THE NUCLEAR FUEL CYCLE:

The radioactivity induced by neutron capture is the major source of radioactive waste that requires management at the power reactor site. This neutron capture occurs in the corrosion products and other impurities in the coolant circulating through the reactor core, and in the structural components of the reactor that are exposed to high radiation levels.

In addition, small quantities of radioactive fission products occur in the reactor coolant and fuel storage basin water as a result of an occasional fuel cladding failure.  The radioactive contaminants of main concern are fission products resulting from nuclear fission in the fuel elements. These fission products are released at the fuel reprocessing plant, where the fuel cladding is either chemically or mechanically breached and the fuel is dissolved.  The soluble fission products dissolve into an aqueous solution, along with the Plutonium and unburned uranium. Gaseous fission products also are released to the plant off-gas systems during the breaching and dissolving operations.

The radionuclides remaining in the solution after recovery of the uranium and plutonium are the fission products and the actinides, also called the transuranics, which include the unrecovered plutonium and the heavier elements (Np, Am, Cm, etc.) of the actinide series formed by neutron capture. Since these transuranics decay through alpha emission, the collective term alpha-emitters are commonly used. Waste contaminated with the alpha emitters is called alpha-contaminated or alpha-bearing waste.

The fission products and alpha-emitters contaminate all materials with which they come in contact and this contamination is passed along the contact chain. Hence, the formation of radioactive waste from what otherwise would be normal industrial waste. The extent to which such contaminated materials can be suitably decontaminated for some purpose only results in additional radioactive waste arising from the decontamination process.

The materials handling and fabrication involving liquid and solid forms of fission products and transuranics, especially for plutonium in the fabrication of plutonium and uranium plutonium oxide fuels, results in contaminated equipment and ventilation systems, all to be dealt with.

2.     NUCLEAR FUEL CYCLE OPTIONS:

The nuclear fuel cycle includes several nuclear fuel cycles may be considered, depending on the type of reactor and the fuel used and whether or not the irradiated fuel is reprocessed and the nuclear material is recycled.  There are two fuel cycle options:

a)     The Open Fuel Cycle (Without reuse of nuclear materials):  The open fuel cycle is the mode of operation in which the nuclear material passes through the reactor just once.  After irradiation, the fuel is kept in at-reactor pools until it is sent to away from-reactor storage.  It is planned that the fuel will be conditioned and put into a final repository in this mode of operation. This fuel cycle strategy is the one currently adopted by many nuclear power countries.  However, no final repositories for spent fuel have yet been established.   This strategy is definitely applied today for Pressurized Heavy Water Reactors (PHWR) and Graphite Moderated Light Water Cooled Reactors (RBMK); and

b)    The Closed Fuel Cycle (With reuse of nuclear materials):  The closed fuel cycle is the mode of operation in which, after a sufficient cooling period, the spent fuel is reprocessed to extract the remaining uranium and plutonium from the fission products and other actinides. The reprocessed uranium and plutonium is then reused in the reactors. This recycle strategy has been adopted by some countries mainly in Light Water Reactors (LWR) in the form of mixed oxide (MOX) fuel.

Apart from the current LWR recycling experience, another closed fuel cycle practice is the recycle of nuclear materials in fast reactors in which, reprocessed uranium and plutonium are used for production of Fast Reactor (FR) fuel.  By suitable operation, such a reactor can produce more fissile plutonium than it consumes.

The nuclear fuel cycle starts with uranium exploration and ends with disposal of the materials used and generated during the cycle.  For practical reasons the cycle has been further subdivided into two stages: the front-end and the back-end.  The nuclear fuel cycle is then completed by the addition of irradiation of nuclear fuel and other related industrial activities to those two main stages.  The front-end of the fuel cycle occurs before irradiation and the back-end begins with the discharge of spent fuel from the reactor.  The specific steps or processes and the corresponding nuclear fuel cycle facilities can be subdivided on front-end, irradiation/nuclear power reactor operation, back-end, and related industrial activities.

3.     NUCLEAR FUEL CYCLE STAGES:

The below sections give the list of stages and processes involved:

a)     The Front-end Processes:  The front-end processes involve some of the steps below:

  • Uranium ore exploration: activities related to the finding and development of the uranium ores for uranium production;
  • Uranium ore mining: activities related to the extracting uranium ore from the ground;
  • Uranium ore processing: activities related to the milling and refining of the ore to produce uranium concentrates including in-situ leaching (commonly called yellow cake — ammonium diuranate containing 80 to 90 percent of U3O8);
  • Conversion: activities related to the refining and conversion to the form which is suitable for any of the other processes;
  • Enrichment: activities related to the isotopic enrichment of UF6 to obtain the appropriately enriched 235U concentration; and
  • Uranium fuel fabrication: activities related to the production of nuclear fuel to be inserted in the nuclear reactor.

b)    The Irradiation/Nuclear Reactor Operation:  The fuel is inserted in the reactor and irradiated. Nuclear fission takes place, with the release of energy. The length of irradiation of a fuel load is in general three to five years in LWRs and one year in GCRs and PHWRs.

c)     The Back-end Processes: The back-end processes involve some of the steps below:

  • At-reactor (AR) spent fuel storage: activities related to the storage of spent fuel in at reactor spent fuel storage facilities (wet type) for interim period. The storage is by definition an interim measure;
  • Away From Reactor (AFR) spent fuel storage: activities related to the storage of spent fuel in away-from-reactor spent fuel storage facilities (wet or dry type) for interim period;
  • Spent fuel reprocessing and recycling: activities related to the special treatment of spent fuel to be able to extract the usable materials and to recycle them in the reactors;
  • Spent fuel conditioning: activities related to the production of spent fuel packages suitable for handling, transport, storage and/or disposal; and
  • Disposal of spent fuel: activities related to the emplacement of spent fuel/wastes in an appropriate facility without the intention of retrieval.

4.     THE TYPICAL NUCLEAR FUEL CYCLE:

Here is a graphical illustration of the cycle:Slide1Here is an overview of the operations/processes associated with the nuclear fuel cycle:

4.1       Uranium Mining:

Uranium is about 500 times more abundant than gold and about as common as tin. It is present in most rocks and soils as well as in many rivers and in sea water.

The largest known resources of uranium ore are in Australia, Canada and Kazakhstan.  Like other minerals uranium is mined in open pit or underground mines. In some cases the uranium is leached directly from the ore without mining. The concentration of uranium in the ore could be from 0.03 up to 20 percent.

Generally speaking, uranium mining is no different from other kinds of mining unless the ore is very high grade. In this case special mining techniques such as dust suppression, and in extreme cases remote handling techniques, are employed to limit worker radiation exposure and to ensure the safety of the environment and general public.

Searching for uranium is in some ways easier than for other mineral resources because the radiation signature of uranium’s decay products allows deposits to be identified and mapped from the air.

While uranium is used almost entirely for making electricity, a small proportion is used for the important task of producing medical isotopes.  Some is also used in marine propulsion, especially naval.

Uranium is a naturally occurring element with an average concentration of 2.8 parts per million in the Earth’s crust. Traces of it occur almost everywhere. It is more abundant than gold, silver or mercury, about the same as tin and slightly less abundant than cobalt, lead or molybdenum. Vast amounts of uranium also occur in the world’s oceans, but in very low concentrations.

Uranium mines operate in some twenty countries, though in 2011 some 52 percent of world production comes from just ten mines in six countries, these six countries providing 85 percent of the world’s mined uranium.  Most of the uranium ore deposits at present supporting these mines have average grades in excess of 0.10 percent of uranium – that is, greater than 1000 parts per million.  In the first phase of uranium mining to the 1960s, this would have been seen as a respectable grade, but today some Canadian mines have huge amounts of ore up to 20 percent U average grade. Other mines however can operate successfully with very low grade ores, down to about 0.02 percent U.

Some uranium is also recovered as a by-product with copper, as at Olympic Dam mine in Australia, or as by-product from the treatment of other ores, such as the gold-bearing ores of South Africa, or from phosphate deposits such as Morocco and Florida. In these cases the concentration of uranium may be as low as a tenth of that in orebodies mined primarily for their uranium content.  An orebody is defined as a mineral deposit from which the mineral may be recovered at a cost that is economically viable given the current market conditions.  Where a deposit holds a significant concentration of two or more valuable minerals then the cost of recovering each individual mineral is reduced as certain mining and treatment requirements can be shared. In this case, lower concentrations of uranium than usual can be recovered at a competitive cost.Slide2Thorium is a possible alternative source of nuclear fuel, but the technology for using this is not established. Thorium requires conversion to a fissile isotope of uranium actually in a nuclear reactor. However, supplies of thorium are abundant, and the element currently has no commercial value. Accordingly, the amount of resource is estimated rather than directly measured as with uranium.

4.2       Milling:

Milling, which is generally carried out close to a uranium mine, extracts the uranium from the ore. Most mining facilities include a mill, although where mines are close together, one mill may process the ore from several mines. Milling produces a uranium oxide concentrate which is shipped from the mill.  It is sometimes referred to as “yellowcake” and generally contains more than 80 percent uranium.  The original ore may contain as little as 0.1 percent uranium or even less.

In a mill, uranium is extracted from the crushed and ground-up ore by leaching, in which either a strong acid or a strong alkaline solution is used to dissolve the uranium oxide. The uranium oxide is then precipitated and removed from the solution.  After drying and usually heating it is packed in 200-litre drums as a concentrate, sometimes referred to as “yellowcake”.

The remainder of the ore, containing most of the radioactivity and nearly all the rock material, becomes tailings, which are emplaced in engineered facilities near the mine (often in a mined out pit). Tailings need to be isolated from the environment because they contain long-lived radioactive materials in low concentrations and toxic materials such as heavy metals; however, the total quantity of radioactive elements is less than in the original ore, and their collective radioactivity will be much shorter-lived.Slide34.3       Conversion:

Natural uranium consists primarily of two isotopes, 99.3 percent is U-238 and 0.7 percent is U-235. The fission process, i.e. the process by which energy in the form of heat is released in a nuclear reactor, mainly takes place in U-235. Most nuclear power plants today therefore require fuel with U-235 enriched to 3-5 percent.  To increase the concentration of U-235, yellow cake is first converted to a gaseous form, uranium hexafluoride gas (UF6) at a conversion facility.Slide4After the yellowcake is produced at the mill, the next step is conversion into pure uranium hexafluoride (UF6) gas suitable for use in enrichment operations. During this conversion, impurities are removed and the uranium is combined with fluorine to create the UF6 gas. The UF6 is then pressurized and cooled to a liquid. In its liquid state it is drained into 14-ton cylinders where it solidifies after cooling for approximately five days. The UF6 cylinder, in the solid form, is then shipped to an enrichment plant. UF6 is the only uranium compound that exists as a gas at a suitable temperature.

One conversion plant is operating in the United States:  Honeywell International Inc., in Metropolis, Illinois.  Canada, France, United Kingdom, China, and Russia also have conversion plants.

As with mining and milling, the primary risks associated with conversion are chemical and radiological. Strong acids and alkalis are used in the conversion process, which involves converting the yellowcake (uranium oxide) powder to very soluble forms, leading to possible inhalation of uranium. In addition, conversion produces extremely corrosive chemicals that could cause fire and explosion hazards.

4.4       Enrichment:

Uranium is enriched in U-235 by gaseous diffusion or centrifuge technology.  Both of these processes work on the principle of separating the lighter U-235 from the heavier U-238, when in the form of uranium hexafluoride gas.

Some reactors, for example the Canadian-designed Candu and the British Magnox reactors, use natural uranium as their fuel, but most types require uranium enriched to 3 to 5 percent U-235.Slide5a)       Gaseous Diffusion Enrichment:  The diffusion process involves forcing uranium hexafluoride gas under pressure through a series of porous membranes. As U-235 molecules are lighter than the U-238 molecules they move faster and have a slightly better chance of passing through the pores in the membrane. The UF6 which diffuses through the membrane is thus slightly enriched, while the gas which did not pass through is slightly depleted in U-235.

This process is repeated many times in a series of diffusion stages called a cascade. The enriched UF6 product is withdrawn from one end of the cascade and the depleted UF6 is removed at the other end. The gas must be processed through some 1400 stages to obtain a product with a concentration of 3 percent to 4 percent U-235.

At present the gaseous diffusion process accounts for about 40% of world enrichment capacity. However, because they are old and energy-inefficient, most gaseous diffusion plants are being phased out over the next five years and the focus is on energy-efficient centrifuge enrichment technology which will replace them.

b)      Centrifuge Enrichment:  Like the diffusion process, the centrifuge process uses UF6 gas as its feed and makes use of the slight difference in mass between U-235 and U-238. The gas is fed into a series of vacuum tubes, each containing a rotor one to two metres long and 15-20 cm diameter. When the rotors are spun rapidly, at 50,000 to 70,000 rpm the outer wall of the spinning cylinder moves at between 400 and 500 meters per second to give a million times the acceleration of gravity. Centrifugal force causes the heavier molecules with U-238 increase in concentration towards the cylinder’s outer edge.  There is a corresponding increase in concentration of U-235 molecules near the centre.

The enriched gas forms part of the feed for the next stages while the depleted UF 6 gas goes back to the previous stage. Eventually enriched gas is drawn from the cascade at the desired concentration and depleted uranium is removed for storage.

4.5       Fuel Fabrication:

Nuclear reactors are powered by fuel containing fissile material. The fission process releases large amounts of useful energy and for this reason the fissioning components – U-235 and/or Pu- 239 – must be held in a robust physical form capable of enduring high operating temperatures and an intense radiation environment.  Fuel structures need to maintain their shape and integrity over a period of several years within the reactor core, thereby preventing the leakage of fission products into the reactor coolant.Slide6The standard fuel form comprises a column of ceramic pellets of uranium oxide, clad and sealed into zirconium alloy tubes.  For Light Water Reactor (LWR) fuel, the uranium is enriched to various levels up to about 4.8 percent U-235. Pressurised Heavy Water Reactor (PHWR) fuel is usually un-enriched natural uranium (0.7 percent U-235), although slightly-enriched uranium is also used.

The fabrication of fuel structures – called assemblies or bundles – is the last stage of the front end of the nuclear cycle shown in Figure: 48-01. The process for uranium-plutonium mixed oxide (MOX) fuel fabrication is essentially the same – notwithstanding some specific features associated with handling the plutonium component.

4.6       Electricity Generation:

Inside a nuclear reactor the nuclei of U-235 atoms split (fission) and, in the process, release energy.  This energy is used to heat water and turn it into steam. The steam is used to drive a turbine connected to a generator which produces electricity. Some of the U-238 in the fuel is turned into plutonium in the reactor core. The main plutonium isotope is also fissile and this yields about one third of the energy in a typical nuclear reactor. The fissioning of uranium (and the plutonium generated in situ) is used as a source of heat in a nuclear power station in the same way that the burning of coal, gas or oil is used as a source of heat in a fossil fuel power plant.

Typically, some 44 million kilowatt-hours of electricity are produced from one tonne of natural uranium. The production of this amount of electrical power from fossil fuels would require the burning of over 20,000 tonnes of black coal or 8.5 million cubic metres of gas.

An issue in operating reactors and hence specifying the fuel for them is fuel burn-up. This is measured in gigawatt-days per tonne and its potential is proportional to the level of enrichment. Hitherto a limiting factor has been the physical robustness of fuel assemblies, and hence burn-up levels of about 40 GWd/t have required only around 4% enrichment. But with better equipment and fuel assemblies, 55 GWd/t is possible (with 5 percent enrichment), and 70 GWd/t is in sight, though this would require 6 percent enrichment. The benefit of this is that operation cycles can be longer – around 24 months – and the number of fuel assemblies discharged as used fuel can be reduced by one third. Associated fuel cycle cost is expected to be reduced by about 20 percent.Slide7As with as a coal-fired power station about two thirds of the heat is dumped, either to a large volume of water (from the sea or large river, heating it a few degrees) or to a relatively smaller volume of water in cooling towers, using evaporative cooling (latent heat of vaporization).

4.7       Spent Fuel Storage:

When used fuel assemblies are removed from the reactor, the used (spent) fuel is hot and radioactive. The spent fuel is therefore stored under water, which provides both cooling and radiation shielding.

Later, spent fuel can be stored dry in shielded buildings or casks. Storage can be either at the nuclear power plant or elsewhere. The heat and radioactivity decrease over time. For instance, after 40 years in storage, the fuel‘s radioactivity will be about a thousand times lower than when it was removed from the reactor. Shielding and cooling will be required for several hundred more years.Slide8In an open fuel cycle spent fuel is regarded as radioactive waste which must be managed and safely deposited. In a closed fuel cycle, spent fuel may be reprocessed to generate more power. Secondary waste from reprocessing, however, must also be managed and deposited.

4.8       Reprocessing:

The spent fuel contains uranium (96 percent), plutonium (1 percent) and waste products (3 percent). The uranium, with less than 1 percent fissile U-235 and the plutonium can be reused as fuel.Slide9Some countries, which have chosen the closed fuel cycle, chemically reprocess spent fuel to separate the useable material, i.e. uranium and plutonium, from the unusable waste. Reprocessing facilities are large industrial facilities.

Recovered uranium from reprocessing can be returned to the conversion plant for conversion to uranium hexafluoride and subsequent re-enrichment.  Plutonium is mixed with uranium to form new fuel (MOX fuel).

4.9       Mox Fuel Fabrication:

Spent nuclear fuel or high level waste can be safely disposed of deep underground, in stable rock formations such as granite, eliminating the health risk to people and the environment. The first disposal facilities will be in operation around 2020.Slide10Waste will be packed in long-lasting containers and buried deep in the geological formations chosen for their favorable stability and geochemistry, including limited water movement. These geological formations have proven stability over hundreds of millions of years, far longer than the waste is dangerous.

Resources:

  1. Radioactive Waste Management by William L. Lennemann;
  2. IAEA Nuclear Fuel Cycle Information System;
  3. IAEA Introduction to the Nuclear Fuel Cycle;
  4. World Nuclear Association – Uranium Mining;
  5. World Nuclear Association – The Nuclear Fuel Cycle;
  6. US Nuclear Regulatory Commission; and
  7. World Nuclear Association – Conversion and Enrichment.

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